One class of substances which have been the subject of considerable study in recent years because of their electrochromic properties are the 4,4'-bipyridinium salts, also known as "viologens." These salts have the general formula: ##STR1## where R is a substituent group and X.sup.- is any appropriate anion. Several of these salts are colorless in solution but upon passage of a current between electrodes are electrochemically reduced and deposited as a violet film upon the cathode. The electrochemical reaction is reversible thus permitting erasure and rewriting of electrodes in a display application.
One viologen of particular interest because of the intensity and fastness of its color when reduced is 1,1'-di-n-heptyl 4,4'-bipyridinium dibromide, (for which, referring to the above structural formula, R is n-C.sub.7 H.sub.15 and X is Br). However, display cells containing this substance (which will be referred to more concisely as di-heptyl viologen bromide) have been found to have only a short useful life because of spotting and non-erasure of the display electrodes after relatively few write/erase cycles. These problems are discussed in a paper by J. Bruinink et al. (J Electrochem Society December 1977 pp 1854-1858) which attributes them to recrystallization of the deposit. The paper concluded that modified viologens with polymethylene-linked bipyridinium rings and with tetrafluoroborate or perchlorate anions were more promising than di-heptyl viologen bromide for electrochromic displays.
U.S. Pat. Nos. 3,950,077 and 3,961,842 also discuss the so-called "hysteresis" effect or spotting of diheptyl viologen bromide after relatively few cycles and attribute this to the bromide anions. These patents propose, inter alia, that a dihydrogenphosphate anion be substituted for the bromide anion to increase the useful life of the display.
In U.S. Pat. No. 4,117,659, it is proposed that the lifetime of a di-heptyl viologen bromide electrochromic display, which is limited by the onset of incomplete erasure, can be extended by the inclusion of a pH buffer in the solution.
In published German patent application OLS No. 2,814,824 the lifetime of an electrochromic display employing any of a large number of viologens including 1,1'-di-heptyl-4,4'-bipyridinium dibromide is stated to be extended by adding a "complexion," preferably a metal chelate to the solution, which may be an aqueous or a methanol solution.
The prior art includes many other examples of di-alkyl bipyridinium salts as electrochromic substances in solution which do not mention problems of non-erasure. The prior art documents reveal additives or traces in many of these solutions, but no effect upon recrystallization or non-erasure is mentioned.